Summary Ohmic heating is a food processing operation in which heat is internally generated within foods by the passage of alternating electric current. The process enables solid particles to heat as fast as liquids, thus making it possible to use High Temperature Short Time sterilization techniques on particulate foods. Ohmic heating rates are critically dependent on the electrical conductivities of the foods being processed, about which little information is available. This paper reports experiments to determine the changes in electrical conductivity which occur during ohmic heating of some common foods. A number of effects which occur during conventional heating, such as starch transition, melting of fats and cell structure changes, are shown to affect the electrical conductivity. In some cases the presence of an electric field induces enhanced diffusion of cell fluids in the food which increases the rate of change of conductivity with temperature above that found by conventional heating. Preheating is found to increase the electrical conductivity of some foods, making them acceptable for ohmic processing.

Aquifers are vital reserves of drinking water which are under threat from pollution. Particular problems are posed by chlorinated compounds such as pesticides and solvents which native microbial populations are unable to degrade. Pump and treat regimes have proved unsuccessful since pollutants remain adsorbed to sediments but a possible solution is the use of introduced microorganisms to degrade pollutants in-situ. It is suggested that methanotrophs may be suitable candidates. Methanotrophs have an extraordinary range of degradative powers due to the non-specificity of their methane mono-oxygenase enzyme. We have shown that Methylosinus trichosporium OB3b is capable of degrading many common chlorinated pollutants co-metabolically when it is grown in a copper-depleted, oxygen-rich medium at neutral pH. In the subsurface however, such conditions do not exist and cultures grown in a medium made with untreated Cambridge aquifer water have a reduced range of degradative powers compared to similar cells grown in a medium made with distilled water. This means that to use methanotrophs for aquifer clean-up, the cells may need to be cultured above ground in ideal conditions and then introduced by some method of injection or infiltration. This may be possible because the degradative reactions are not coupled to growth and Methylosinus trichosporium OB3b cells maintain pollutant degrading ability up to 19 days after they have stopped growing. A suspension of these cells may thus be treated as a biocatalyst.

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